Many industrial fields require laser processing capability and for such applications, the primary concern is often to generate optical laser pulses with, to some extent, real-time control over the pulse amplitude, duration, shape, peak power and repetition rate. In some applications, such as laser-based material-processing, the rise time and fall time of the shaped optical pulses are also important functional specifications.
U.S. Pat. No. 7,348,516 (SUN et al.), entitled “Methods of and laser systems for link processing using laser pulses with specially tailored power profiles” presents many arguments in favour of pulsed laser systems providing fine control over the pulse temporal power profile in the nanosecond regime, for facilitating better link process quality and yield. Three different laser architectures providing a certain control over the laser pulse shapes are described therein. U.S. Pat. No. 7,126,746 (SUN et al.) further teaches a laser system providing control over the pulse shapes and having a Master Oscillator Power Amplifier (MOPA) configuration.
U.S. Pat. No. 6,281,471 (SMART), entitled “Energy-efficient, laser-based method and system for processing target material” describes many requirements and specifications concerning the temporal generation of square laser pulse shapes in material processing. The system presented therein includes, among its main components, a controller for generating a processing control signal, and a signal generator for generating a modulated drive waveform based on the processing control signal.
Optical pulse shaping implementation can originate from digital electronic means, where some electronic apparatus reads a given sequence of digital samples previously stored in a memory buffer, and writes these samples into a digital-to-analog converter (DAC). The shaped analog signal output by the DAC is then fed to a buffer amplifier having enough bandwidth and drive capability for directly modulating a light source such as a laser diode, or driving an electro-optic modulator.
One challenge which arises in pulsed laser systems providing a great flexibility over the conditions of operation (repetition rate, pulse shape, etc.) is that transient effects often occur in the laser system when these conditions of operation modify the population inversion in gain media found in this system, such as the gain medium of a laser diode or one used as an amplifier. For example, when the repetition rate of a laser is suddenly reduced, a number of pulses following the transition will have more energy than the steady state pulse energy at the modified repetition rate. It can be important to mitigate these transient effects in order to avoid detrimental consequences for a given application using the laser output. For example, in semiconductor memory processing applications where a single pulse is used to process a given structure, such as severing conductive links for memory repair, it is important to keep the pulse energy within a well defined energy per pulse process window. If the pulse energy is too low, then the link may be incompletely removed.
In cases where the energy per pulse exceeds the allowable energy process window, excess pulse energy may be coupled into adjacent or underlying link structures, or the substrate itself, causing highly undesirable damage to the device. In multiple pulse laser processes, such as laser drilling of microvias in semiconductors, or laser scribing of thin film photovoltaic devices, it is important that successive pulses remain substantially uniform in order to produce laser processed features that possess the desired dimensions and feature surface quality. The control of the laser transient response is also important in laser surgery, as the amount of energy deposited in living tissues must be accurately controlled in order to avoid inducing damages to the neighbouring tissues.
High value is placed upon the throughput of work pieces satisfactorily produced by a laser processing system. Therefore, methods to achieve pulse stabilization in lasers, particularly in lasers employed in laser processing or medical laser systems, are highly desirable. When laser processing device target structures whose layout requires laser processing pulses to be emitted at interpulse periods which are not fixed, methods for generating laser processing pulses with substantially equal pulse amplitude and energy per pulse are highly desirable. Industrially important laser applications, such as laser repair of dynamic random access memory (DRAM), laser scribing of photovoltaic cells, and laser drilling of microvias in semiconductor, flexible interconnects, IC packages, dielectrics, including glass, and metals, typically require pulsed laser output characterized by more than one characteristic interpulse period. As those skilled in the art will appreciate, q-switched solid state lasers and pulsed fiber lasers are commonly employed in these and similar laser processing applications in which laser processing pulses are required to be emitted under such conditions. Therefore, improved techniques for pulse stabilization of advanced pulse laser sources are of substantial interest and value to practitioners in such industries.
Previous workers have described methods for mitigating pulse transients, such as actively controlling the pump power level in the gain medium of a solid state medium so as to control the population inversion dynamics. For example, DAVENPORT et al, in U.S. Pat. No. 5,151,909 disclose laser systems having programmable pump power modes to actively control the amount of power delivered by solid state lasers. In one embodiment disclosed by DAVENPORT et al, the pump control capability solves a problem for transitions between different modes of operation of the laser. However, for several material processing applications where the laser beam quality is critical, such an approach would bring detrimental beam quality fluctuations and beam pointing issues induced by thermal changes in the laser system, originating from varying gain medium pumping conditions.
CHAN et al in U.S. Pat. No. 5,226,051, entitled “Laser Pump Control for Output Power Stabilization” teach a method for modulating the pump power supplied to the laser in a diode-pumped q-switched solid state laser, such that pulses emitted with differing interpulse periods will possess comparable pulse energy and pulse amplitude. In this method, the pump power is delivered at a first high value for a time τr. If the laser receives a signal within a t<τr, then a laser pulse is emitted by the laser with the energy stored during time t. In the case where t>τr, then the pump current is reduced to a pre-programmed lower value, which may be time dependent, to maintain the stored energy at a pre-determined level to keep the laser pulse output at a specific value. As those skilled in the art will appreciate, this method can advantageously produce a laser pulse output with substantially equal pulse energy and pulse amplitudes over a wide dynamic range of interpulse periods by substantially equalizing the stored energy independently of the interpulse period. As those skilled will further recognize, a key limitation of this technique is the maximum energy per pulse that may be produced, as the energy per pulse and pulse amplitude substantially correspond to that produced at the shortest interpulse period. This limitation on the maximum energy per pulse becomes increasingly severe with increasing pulse repetition frequency (see Koechner, “Solid-State laser engineering”, Springer-Verlag, Chap. 8. FIG. 8.8). For maximum Pulse Repetition Frequencies (PRF) substantially greater than about 30 KHz, the resultant efficiency losses introduced by the method of CHAN et al are unacceptable for most industrial applications.
For laser applications requiring very high throughput of laser processed workpieces, methods which employ pulse picking techniques are commonly employed to achieve stable laser processing output are often employed. BAIRD et al in U.S. Pat. No. 6,172,325, “Laser Processing Power Output Stabilization Apparatus and Method Employing Processing Position Feedback”, describe the use of a q-switched solid state laser which operates in cooperation with a pulse processing control system that employs an autopulse mode and a pulse on position mode to stabilize the laser output delivered to dynamic target locations on a workpiece moved by a positioner. To overcome the deficiencies of the prior art of CHAN et al, the laser pulses are consistently emitted at a near maximum PRF in an autopulse mode in order to maintain a stable laser output. In the autopulse mode, an external optical modulator, such as an acousto-optic modulator, blocks the laser output and prevents the pulses from reaching the workpiece targets. In the pulse-on-position mode, the laser emits a pulse each time the beam position moves through a target workpiece coordinate position. The processing control system commands the external optical modulator to an “open” position whenever the target workpiece coordinate position matches a position requiring a process pulse to be transmitted to the target workpiece. The essence of this technique is that it provides a near constant interpulse period for the laser, thereby resulting in stable pulse output for processing of target workpieces. As can be appreciated by those skilled in the art, this technique is particularly suitable for the employment of lasers utilizing harmonic output since the harmonic output from a frequency converted laser is very sensitive to any variations in the fundamental input to the nonlinear conversion elements, including pulse repetition frequency and interpulse period variations.
For laser processing of target structures arrayed periodically, such as DRAM link structures, it is often useful to employ pulse triggering techniques which cause the emission of one or more process correction pulses for the purpose of introducing temporal offsets to the process interpulse period. BRULAND et al, in US Patent Application No. 2007/0228024 A1, “Methods and systems for decreasing the effective pulse repetition of a laser” describe techniques for emitting various types of process correction pulses, referred to by BRULAND et al as dummy pulses and pre-pulses. A dummy pulse is a pulse triggered coincidentally with a target position but which is blocked by an external optical modulator from propagating to strike target on the workpiece, for example for structures where only selective targets are to be actually processed. Typically, dummy pulses are blocked by an external optical modulator, such as an acousto-optical modulator or electro-optical modulator, from propagating to strike the target structures on the workpiece.
The purpose of dummy pulse emission is to maintain laser emission at a near constant PRF=velocitystage/Δxtarget pitch. However, as described by BRULAND et al, for the case of laser processing of memory links, beam positioning requirements to move to neighboring link banks may require changes in stage velocity. In addition, other positioning offsets may be present. Therefore, to achieve stable laser process pulse output, the laser may be commanded to emit non-process pre-pulses in addition to dummy pulses and process pulses. The key attribute of non-process pre-pulses is to introduce a positive or negative interpulse time offsets which yield emission of dummy pulses and process pulses at an interpulse time=1/PRFprocess, thereby substantially improving the uniformity of the energy per pulse and pulse amplitude of the process pulses.
Intrinsic to the methods described by BAIRD et al and BRULAND et al is the use of a pulsed laser source capable of producing stable pulse output following a dynamic change in the interpulse period. BAIRD et al teach the use of Q-switched solid state lasers, including diode-pumped Q-switched solid state lasers and further teach that lasers which produce harmonic converted outputs may be advantageously employed. BRULAND et al similarly teach the use of a Q-switch laser in some embodiments. In another embodiment, BRULAND et al describe employing a semiconductor laser in a pulsed fiber laser configured in a master oscillator power amplifier configuration. In this embodiment, BRULAND et al mention that a semiconductor laser employed as a master oscillator to pump the gain fiber in the master oscillator power amplifier configuration can be controlled to create the pre-pulse and process pulse at the appropriate times. BRULAND et al provide no further teaching concerning the construction or operation of such a configuration.
In spite of the advances in the art described above, there remains a need for improved methods to provide efficient transient response in the field of pulsed laser oscillators and pulsed laser oscillator-amplifiers which are capable of substantial pulse shape and pulse repetition rate flexibility.